This invention relates to a hydraulically actuated brake comprising at least one friction element, for example a brake pad in the case of a disc brake, adapted to be clamped against a rotating track by a piston fitted slidably in a cylindrical body in response to a build-up of pressure in an actuating chamber containing hydraulic fluid and defined in part by the said piston.
The invention relates particularly to such a brake in which the piston comprises a cup-shaped element having a cylindrical wall sliding in the cylinder and a base adjacent the actuating chamber.
The cup forming the piston is usually made from metal for example, steel, and is applied directly to the friction element. During braking, the heat generated by the friction between the friction element and the rotating track causes a rise in the temperature of the metal piston and consequently of the hydraulic fluid which is contained in the actuating chamber and in contact with the base of the cup. There is a danger that the hydraulic fluid, which generally consists of oil, may lose its operating efficiency when the temperature exceeds a given value, for example 150.degree. C. It is therefore important that the heat generated by friction should not cause the temperature of the hydraulic fluid to rise above such a value.
The invention relates particularly to such a brake in which means are provided to prevent an excessive rise in the temperature of the hydraulic fluid consisting of a cylindrical core of mechanically strong and thermally insulating material fitted inside the cup forming the piston, the walls of the core and of the cup being fixed together by connecting means having a press fit.
The side walls of the core and of the cup are generally continuous, which permits a direct and effective press fit, but only under assembly conditions which necessitate high precision and which are in any case difficult because of the large surface areas present. Moreover, the core frequently forms an insufficient heat shield to prevent the temperature of the hydraulic fluid from rising to an excessive value.
The object of the invention is a hydraulically actuated brake of the kind described above, in which the hydraulic fluid of the actuating chamber is protected in a particularly effective manner from an excessive rise in temperature, and the assembly conditions for the press fit of the core in the cup are improved.